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Bridged vs Conventional Amps
Bridged vs Conventional Amps
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Old 7th August 2009, 11:30 PM   #11
Nelson Pass is offline Nelson Pass  United States
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Bridged vs Conventional Amps
You can get reduced power supply noise, better common-mode noise
rejection, twice the slew rate, cancellation of 2nd harmonic and
lower (safer) voltages.

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Old 8th August 2009, 12:00 AM   #12
RocketScientist is offline RocketScientist  United States
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Quote:
Originally posted by Nelson Pass
You can get reduced power supply noise, better common-mode noise
rejection, twice the slew rate, cancellation of 2nd harmonic and
lower (safer) voltages.
Ok, I agree with all of those except possibly common mode noise (it depends on where such noise enters into the signal path).

But, none of them AFAIK, will reduce real world distortion unless you're measuring *only* the 2nd harmonic. Given that 2nd harmonic distortion is generally much less of a problem (both subjectively and objectively) than the 3rd and higher orders, improvements there don't help the overall distortion or sound quality situation much. And I would expect the other factors I mentioned would more than offset any 2nd harmonic gains (i.e. the bridged amp having more 3rd and higher order distortion).

Quote:
Originally posted by jcx
MOSFET or thermal limited Bipolar SOA devices come out about the same in total number of output Q for given output power
OK, are you saying for a given power output you need roughly the same amount of output devices in a bridged vs single ended design? I realize the transistors are in a much better part of the SOA curve in a bridged amp compared to a conventional amp of the same power. But that's offset by the peak current being twice as high, and also of course, by the need for two mirrored output stages instead of one.

I haven't done the full analysis, but I would think it comes to how much output voltage and peak current you want. For example, I'm fairly sure a 200 watt/8 ohm bridged amp would need more output devices total if you want the same 2 ohm load performance as a conventional 200 watt/8 ohm amp. For a 1000 watt amp, however, it might be the other way around.

It would seem, at least based on commercial designs, there's a crossover point at some power level where the advantages of a bridged design overcome their disadvantages. But I've not seen any hard data to back that up or identify at what power level that might be true.
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Old 8th August 2009, 02:17 AM   #13
RocketScientist is offline RocketScientist  United States
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I realized another potential advantage to the "grounded bridge" and similar topologies where the output stage power supply is not ground referenced: Such amps can have quieter grounds for the input stages of the amplifier. If the output stage power supply is floating, it avoids all the high ground currents in a conventional amp. As with SOA, this benefit is proportional to the output power of the amp.

Some also claim because the current drawn from the power supply in a bridged amp is full wave, vs half wave in a conventional amp, the bridged amp produces less induced (i.e. EMI coupled) noise in the rest of the amplifier. I'm less convinced this really helps. I'm not sure what the spectral energy differences are between the two waveforms, but I would suspect the twice higher higher currents in the bridged amp might more than offset the full wave advantage?
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Old 8th August 2009, 04:20 AM   #14
CBS240 is offline CBS240  United States
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Quote:
Originally posted by Nelson Pass
You can get reduced power supply noise, better common-mode noise
rejection, twice the slew rate, cancellation of 2nd harmonic and
lower (safer) voltages.


I agree.

Also it depends on how accurate the phase splitting section of the circuit is, because if there is phase shift away from 180 degrees even at higher frequencies, there will be a related distortion. no transformers allowed here, eh?


The PS has to be more robust in terms of current and uF's of capacity, but they can be lower voltage. Not a terrible difference in cost. Larger traces and thicker wires, eh, big deal.

I tend to favor the idear of using a common, balanced bridge VAS to drive each OPS instead of two separate VAS circuits. The better CMRR is a plus. In one version of my insane complex bridge amp, I drove the amp to death, deliberately, to see what would happen. One of the N-ch OPT melted short, connecting that speaker lead to the positive rail. The reaction of the DC servos that control the common mode amplifier part then attempts to correct for the CM error by trying to swing that output negative. The result was the other output phase was driven to the positive rail resulting in no DC current flow in the speaker and the circuit locked up in a steady state. This 'auto-protect' feature was an interesting observation I doubt could be achieved easily with a single end output. Hopefully sometime soon I will get some time to play around with it a bit more. I have quite a few experiments and tests I want to do yet.
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Old 8th August 2009, 04:58 AM   #15
LineSource is offline LineSource  United States
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Default Balanced outputs vs. bridged amplifiers

Balanced outputs vs. bridged amplifiers

From Pass Literature:
"The Super-Symmetric X amplifier design exploits the symmetry of a matched balanced amplifier so that distortion and noise are cancelled at the output to the loudspeaker, then uses a small dose of a unique new form of feedback to make that symmetry more perfect.

The topology takes advantage of the character of specially matched balanced amplifiers that are cross-coupled to provide cancellation of distortion and noise. The result provides high performance with very simple linear circuits, better than previous similar efforts by an order of magnitude. It was named Super-Symmetry as an homage to particle physics, but it is popularly known as the X circuit.

Balanced amplifiers improve performance by differentially rejecting distortion and noise. To the extent that distortion and noise are identical, they vanish at the output, typically by a factor of 10 or so for matched single-ended Class A circuits.
Super-Symmetry extends this concept by using feedback only to make the distortion and noise more identical on each half of a balanced circuit, not to eliminate it as such. This gives as much as a 100:1 reduction in unwanted distortion and noise without requiring the equivalent amount of negative feedback. It is simply much easier to tweak the two halves of the circuit into identical symmetry than to eliminate all the distortion in each half of the circuit."

My ears like the sound of bipolar output Super-Symmetric amplifiers. I do not have any balanced drive music sources and use a single ended source input to generate balanced outputs.
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Old 8th August 2009, 08:04 AM   #16
tiefbassuebertr is offline tiefbassuebertr  Germany
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Quote:
Originally posted by Nelson Pass
You can get reduced power supply noise, better common-mode noise
rejection, twice the slew rate, cancellation of 2nd harmonic and
lower (safer) voltages.

But a major disadvantage should not be unmentioned: The requirement to a small degree of deviation from the two amplifier halves is high and is easiest to realize by CSPP (Circlotron).
Therefore sometimes I observed a lack by quality of power amplifiers in the upper frequency range, when they switched from stereo to mono. I experienced this by certain, individual devices of the NAD power amp 2600 and 2400. The reason was mainly not selected output power devices from Toshiba - so I think
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Old 8th August 2009, 08:35 AM   #17
Zero D is offline Zero D  United Kingdom
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Hi,

In theory you can get 4 x the power into the same load, that's if the power supply is beefy enough of course. So that in itself is an attractive proposition.

This also means the voltage rails can be lower, with the benefit of reduced thermal stress, and cooling requirements, and lower voltage output devices.

I think a correctly balanced input and output is hard to beat.
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Old 8th August 2009, 01:00 PM   #18
megajocke is offline megajocke  Sweden
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Here's a fun circuit which is grounded bridge:

http://media.qscaudio.com/pdfs/disco...s/PL9.0PFC.pdf

schematic:
http://www.qscaudio.com/support/libr...s/pl9.0PFC.pdf

4500W / channel, 2 ohms

The 4 step class H would have needed eight power supply rails and six rail switches if it had been a non-bridged amplifier. Of course, now the power supplies for the two channels need to be separate, but having one channel working in case of PSU failure is probably worth it in this application.
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Old 8th August 2009, 02:15 PM   #19
KSTR is offline KSTR  Germany
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One specific benefit of bridging comes with true class-A operation of the halves. Then the total current draw from the supply is constant, the amp cannot modulate the supply with load currents. Still the supply should be low ripple and low and flat impedance of course. Since there are no load currents in the GND, a single supply can be used and a reference GND derived from that in a way that it optimzes PSRR for the individual halves right to start with.

Another point would be, for low/zero FB designs, that one can trade some even harmonics to favor low amounts of odd ones, as the former are cancelled by the bridge.

- Klaus
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Old 8th August 2009, 02:15 PM   #20
djk is offline djk
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"My ears like the sound of bipolar output Super-Symmetric amplifiers. I do not have any balanced drive music sources and use a single ended source input to generate balanced outputs."

Hadley 622 (60's vintage):

Click the image to open in full size.
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